The invention relates to a micro-scale firetrain (MSF) for MEMS (microelectromechanical system) based electromechanical safety and arming (S&A) devices. The invention significantly reduces the volume and cost over similar-functioning macro-scale initiating firetrains, while also reducing the amount of lead-bearing compounds associated with the firetrain.
The principal application of the invention is in munition fuze safety and arming for gun-launched and other tube-launched munitions, wherein, typically, launch (setback) acceleration and spin-induced centrifugal acceleration are sequentially detected, thresholded, and utilized to effectuate mechanical arming of a firetrain. More generally, this invention applies to any safety-critical explosive device or system, such as an ammunition fuze safety and arming device (S&A), in which the explosive firetrain mass, volume, and firing energy need to be minimized and the explosive firetrain needs to be kept in a safe condition prior to intended firing. Other applications of the present invention are for safety and arming in grenades, mortars, tank rounds, rockets and submunition grenades, mines, torpedoes and other weapons.
U.S. Pat. Nos. 3,686,934, 3,759,183, 3,812,783, 4,644,863 and 6,497,180 generally describe an electrically-initiated detonator (in particular, U.S. Pat. No. 3,686,934) in the form of a cylindrical enclosure (a cup), formed with one end closed and the other end open, filled with a first layer of secondary explosive, a second layer of primary explosive, and topped with an explosive header that is crimped in place to seal the cup. The header has a spot-welded heater wire (bridgewire), which is generally painted with a sensitive explosive primary mix for initiation, and contact electrodes of some kind. The detonator is then used in a mechanical S&A system wherein it functions as an explosive initiation member between an initiation stimulus (fire pulse from fuze circuit) and an acceptor charge, sometimes called a “lead charge”.
In these prior designs, the sealed assembly including the cylindrical enclosure, the explosive fill, the header and bridgewire, etc., constitutes an electric detonator whose position relative to an input stimulus and an output firetrain is managed by the mechanism of a mechanical safety and arming (S&A) device. Because the detonator is large and massive (compared to the MEMS mechanism-based implementation of the present invention), the S&A mechanism must also be large.
To elaborate further, prior-art electric detonators and explosive initiation systems are all of a similar construction, including a current-pulse generating fire circuit, an electrical-contact header, a resistively-heated hot-wire bridge or thin-film element, a glass or plastic seal between header and cup, and an explosive-filled cup. In general, the header comprises concentric metal contacts electrically coupled through a bridgewire but otherwise electrically separated by a glass or plastic seal from the explosive cup. The bridgewire is painted with a sensitive primary explosive mixture such as lead styphnate. The header is pressed and crimped into a cup that has been previously loaded with first a secondary explosive for output and a primary explosive transfer charge on top of the secondary as described above. The cup generally has a coined bottom membrane. When a current is passed across the contacts through the sealed device, the bridge wire undergoes resistive heating, initiates the first primary explosive, which initiates the second primary explosive, which initiates the output fill, which shears and throws the coined metal cup-bottom at high velocity across a gap into a receiver explosive, which is detonated thereby.
There are variations among the cited prior-art patents from the general case described above. U.S. Pat. No. 3,759,183 combines an instantaneous electric detonator with a delay electric detonator tandemly in a single structure. U.S. Pat. No. 3,812,783 describes a detonator that is initiated optically instead of electrically. U.S. Pat. No. 4,644,863 describes a detonator with an electrically insulated casing and pole piece. Finally, U.S. Pat. No. 6,497,180 describes a detonator in which the heater wire or bridgewire of other designs is replaced by a carbon film resistor. In all these cases the detonators comprise essentially the same overall construction as was noted above. In every case, the metal cup or enclosure is quite large compared to the corresponding movable element of the present invention.
Typically an electric detonator or initiator such as the M100 is assembled as a self-contained unit into a mechanical safe and arm (S&A) system of a weapon, by dropping it into a hole in a slider or rotor that is able to move it—by translation or rotation—to control the mechanical arming status of a firetrain. The S&A mechanism functions by holding the detonator away from (out of line with) downstream elements of a firetrain, such as a so-called lead charge or a booster charge, until a weapon is launched. Upon detecting a valid launch, the S&A mechanism places the detonator in a position in-line with an explosive firetrain, which arms the weapon. Such movement usually involves translation of a slider or rotation of a rotor.
In this process, the whole detonator is moved to and fro as an explosive transfer element. Often the leads are the explosive train elements that are moved from an out of line position to an in line position. The leads are out of line in a rotor that mechanically blocks transfer of the detonator's explosive output to the S&A's output leads; the detonator remains stationary, since it is connected by lead wires to the firing circuit. This manipulation requires a relatively large mechanism, since the typical detonator body is comprised of a cylinder approximately 0.25 inches in length by 0.1 inches in diameter. Some newer detonators are shorter, of approximately 0.19 inches in length, and a little smaller in diameter. Nonetheless, these dimensions and the need for a transport mechanism still larger than the detonator itself set a practical limit on how small a conventional type mechanical S&A system can be made.
The present invention radically revises that physical limit by reducing the size of the movable element in the initiating explosive train. For example, the volume of the movable transfer charge used in the present invention is about 150 times smaller than the analogous movable element, typically a detonator such as the M100. Because the inventive firetrain transfer element is so small, the mechanism necessary to control and move its position for arming can be correspondingly small, whereby miniaturization of the entire S&A device is made possible.
Also, traditional assemblies often require the use of an explosive barrier capable of stopping the output of the whole M100 detonator. This requirement adds considerable mass and the need for a strong structure. A system with the present invention however must only present a barrier or a gap that can stop the output of a far smaller transfer charge, greatly reducing the need for ancillary structure.
In one embodiment, the present invention entails a movable transfer charge that contains no primary (sensitive) explosives and therefore is less hazardous to load, handle, and use, than the M100. This is because during assembly of the present invention into a MEMS S&A structure such as that cited earlier no significant amount of secondary explosive is ever in-line with (juxtaposed against or near) sensitive primary explosives, making it much safer than prior-art S&A assemblies to handle. In addition, the inventive micro-scale firetrain as a whole contains much less primary explosive than the M100 detonator.
Prior-art detonators for S&As are unsatisfactory, compared with the current invention, because they are larger, and require tight tolerances as well; also layered press loads. They typically use expensive-to-manufacture spot-welded bridge wires to initiate the first reaction, whereas the present invention uses wafer-based batch-produced thin-film-bridge chips. The comparatively sizable explosive output of a standard detonator requires a larger distance between the armed/not-armed states inside the mechanical S&A, forcing the assembly to be larger. At the same time, because such an assembly must of necessity have relatively large gaps between conventionally-manufactured working parts, the detonator must be powerful enough to operate across those gaps. Prior art detonators require a larger quantity of primary explosive to be handled on the detonator and fuze assembly line, which increases the safety hazard to production-line personnel. Prior art detonators use a larger quantity of lead-containing primary explosives, which has adverse health and environmental impacts during both manufacture and in-service functioning.
The present invention meets the need for an ultra-miniature firetrain that can be implemented in a MEMS-based safety and arming device or more generally in an ultra-miniature mechanical-logic/explosive-coupled assembly. The invention has the potential to reduce the dependence of the military on traditional detonators such as the M100 electric detonator, which are relatively bulky and expensive. The invention also significantly reduces the output of lead-containing byproducts of the explosive initiation when compared to the output of a conventional detonator such as the M100 electric detonator. One problem solved by the invention is the need for smaller and cheaper S&As to increase weapon lethality and affordability, by virtue of eliminating the standard detonator and implementing the functions of the detonator and mechanical safety logic within the miniature S&A mechanism itself. The invention also provides feasible ultra-miniature explosive components suitable for assembly in an explosive train applicable to the MEMS scale.
Compared to the prior art, the invention has differences that give beneficial results. One principal difference is miniaturization. New applications are possible because the radical miniaturization of the explosive train elements yields a corresponding miniaturization of the S&A mechanism needed to transport the moving transfer charge and also a corresponding reduction in the distance (stroke) necessary to separate safe and armed conditions, hence a much smaller overall structure. The present invention can be adapted to or employed in miniaturizing existing fuze S&As and circuits because the electrical input to initiate the MSF of the current invention is or can be identical to that of the standard M100 or similar detonators.
The invention provides a remedy for applications for safing and arming of miniature munitions where extremely small size is required. The greatly reduced explosive train volume allows for redundant explosive trains thereby increasing reliability. The greatly reduced fuzing volume creates volume for other munition functions/systems. Miniaturization has ancillary effects such as reducing the mass of the firetrain components, and also thereby the mass of the mechanism controlling the firetrain. Mass reduction improves implementations in situations such as impact and penetration environments, high vibration environments, and launch shock environments, in which a large-mass S&A would not only be subjected to potentially destructive loads but in which the large-mass S&A would subject its surrounding structures to potentially destructive loads.
With its greatly reduced size and mass, the MEMS S&A is also much easier to protect from high-G transients. Also, being smaller, the location options of the firetrain are increased. The ease of providing duplicate systems is increased, because the invention occupies only a fraction of the volume of large-scale S&As, thereby increasing explosive system reliability. Increased reliability means decreased dudding rate, which reduces the unexploded ordnance problem. Additionally, in high transient temperature environments, part of the volume freed up can be used to insulate against high temperature transients.
The inventive MSF employs conventional primary explosives upstream of the interrupter/movable transfer charge, but all other charges may be secondary explosives. The primary-explosive initiating charge (later referred to as the “input column”) is stationary with no moving electrical contacts or connections required. The transfer charge (movable firetrain element) of the present invention contains, in one embodiment, only secondary (insensitive) explosives, and therefore may be safer to load, handle, and use than systems containing a movable element (e.g., M100 detonator) that contains primary explosives. Implementation of the present invention may include primary explosives in the transfer charge, if desired for some purpose. Relative to a standard electric-initiated detonator such as the M100 detonator, the invention eliminates more than 96% of the lead-containing primary explosives.
The invention includes advancements in safety in applications for small volume explosive systems where hand safety is required; in applications for small volume explosive systems where total munition safety is required; and in applications for high-G shock environments where minimization of mass is desirable. The movable transfer charge, in one embodiment, contains no primaries (primary, or sensitive, explosives), resulting in safer handling. The present invention (the MSF) classifies as an interrupted explosive train because the MEMS S&A mechanism physically retains the transfer charge out of line with other elements (the donor and the acceptor) of the firetrain until mechanical arming is achieved. The shape of the transfer charge necessitates, in one embodiment, two right-angle turns of the firetrain, which is hard for nature to imitate in the absence of the transfer charge. This is a safer design because the input and output explosive columns are never in-line. Transmission of detonation from input to output requires the presence of the coupling energetic transfer charge.
The present invention also achieves cost savings. There is reduced cost for S&A assembly including elimination of traditional detonator; reduced firing range cleanup costs because of reduced lead in MSF compared to electric detonators; reduced health hazard to manufacturers and firing range personnel because of reduced lead; cost savings from batch process MEMS fabrication/explosive loading/assembly; cost savings from reduced quantities of explosives in firetrain; and cost savings for configuration changes in future from batch nature of MEMS fabrication/assembly. The present invention can be less expensive in large-quantity production because it is amenable to in-situ and even wafer-scale slurry-loading or cast-loading techniques.
The present invention can be used with existing weapon systems such as the IAWS (XM29 Integrated Air Burst Weapon System) 25 mm grenade round fireset using the same firing parameters. The output of the MSF is able to detonate existing output explosive relays. The contemplated use of the MSF in the MEMS S&A complies with the safety requirements of MIL-STD-1316 and MIL-STD-331, Test D1. The MSF components individually and collectively have been shown to survive and function after experiencing launch shock levels of 45,000–65,000 Gs peak launch acceleration.
The invention includes a three-explosive-component out-of-line explosive train including a stationary input charge, a movable transfer charge, and a stationary receiver/output charge, with a total volume of explosives (the three components) less than about 0.002 cubic centimeters. The input and movable elements of the train comprise less than about 0.0012 cubic centimeters. The explosive transfer charge is detonable by an input explosive charge of 0.001 cubic centimeters volume or less. Novel configurations for the separation of explosive components maintains safety in out-of-line (safe) conditions.